It has been proposed--see the referenced Hegeler, U.S. Pat. No. 3,949,401--to radiate with FM transmitters various announcements, for example traffic announcements or the like. Transmitters which are capable of radiating such announcements utilize an auxiliary carrier besides the program modulation. In accordance with one proposal, the auxiliary carrier is radiated at a frequency of 57 kHz which, in FM stereo transmitters, is derived as the third multiple of the 19 kHz stereo pilot tone. It is radiated in synchronism therewith. The 57 kHz subcarrier is phase-locked with the 19 kHz pilot tone, the zero crossings of the two frequencies occurring at the same time. The auxiliary 57 kHz subcarrier is utilized to transmit additional information, hereinafter recognition information, which is applied to the auxiliary carrier in form of amplitude modulation. The overall system is described in the above-identified Hegeler patent, and in the referenced Eilers and Bragas applications.
One of the recognition frequencies is radiated together with the intelligence transmission on the main carrier frequency. This recognition frequency thus indicates that, during its radiation by the FM transmitter, the FM transmitter provides program information of the specific announcement, for example a traffic announcement, sports report, or the like, and the recognition frequency which recognizes the announcement is referred to as the AR frequency. The AR frequency, in accordance with the proposed system, is placed within a very narrow frequency band, in the range, for example, of between 100 to 200 Hz, for example 125 Hz, and modulated on the auxiliary carrier in the form of amplitude modulation.
Receivers which are designed to recognize the AR frequency have a 57 kHz detector and an amplitude demodulator, which is coupled to a switching circuit controlling the audio frequency stage thereof. The control may be used to raise the volume during the announcement, or to provide audio output if the receiver is muted, for example by inhibiting a muting circuit; in combined radio cassette or cartridge recorder-transducer systems, the switching can also be used to transfer reproduction from the cassette or cartridge to the receiver for transmission of the announcement, for example a traffic or emergency announcement, when the announcement starts, and to revert the reproduction by the receiver to cassette or cartridge reproduction after termination of the announcement. Other switching functions can also be controlled thereby.
The auxiliary carrier also radiates recognition frequencies which characterize specific radio stations, or geographical regions. Various transmiters capable of radiating announcements have assigned thereto radio station or region recognition frequencies (RR). If the stations are within a certain region, they may all have the same radio station recognition frequency, that is, the RR frequency assigned thereto, since traffice announcements may relate, essentially, to the same geographic region. The amplitude of the auxiliary carrier is continuously modulated with the respective RR frequency assigned to the station. The bandwidths of the respective RR frequencies, and the relationship with respect to each other of different frequencies are so selected that, with a quality of 20, an adjacent channel damping of more than 15 db is obtained. Various RR frequencies are possible within the available frequency band; the frequencies are so selected that the harmonics of the respective assigned frequencies are far between other assigned frequencies.
During an announcement, the auxiliary 57 kHz subcarrier is thus modulated by two recognition signals: the AR signal to indicate that an announcement is being radiated, and the radio station or region RR signal indicating the particular station. If no announcement is being radiated, the auxiliary subcarrier is modulated only with the RR signal to indicate, upon tuning of a receiver, that the particular station may provide announcements.
Various radio transmitter stations utilize coders which control modulation of a subcarrier with the respective RR and AR signals. The coders, in the past, have been matched to the particular type of transmitter. The respective AR frequencies, and the RR frequencies, can be selected in accordance with governmental or national standards.
Stereo FM transmitters and monophonic FM transmitters usually use different systems to generate an auxiliary carrier. Further, it is desirable that the degree of modulation of the auxiliary carrier, due to the AR signal and to the RR signal, should be different. If individual coders have to be designed and provided for respective transmitters, so that they are specially matched to the transmitter, the cost of the coder will become substantial. This is particularly important if a large number of comparatively low-power transmitters desire installation of the respective coders, since the cost of the coders may become a substantial fraction of overall station costs.